1. Field of the Invention
This invention relates to metal casting and, more particularly, to a composite grained cast article made from a single grained article.
2. Description of the Prior Art
Improvements in gas turbine engine technology have been based, in part, on a utilization of improved high temperature alloys and the condition of the grain structure of such alloys in article form. One of the more widely used types of alloys employed in hot sections of gas turbine engines are those based on the element nickel, or cobalt, or both. Such articles as turbine blades and vanes are conveniently manufactured by the well-known lost-wax type precision casting process. However, the turbine engine designer currently has available to him several microstructures or crystal forms from which to select. Such selection is based on properties desired by the designer and the relative cost of manufacturing such a form of the article. For example, multi-grained castings have been provided with nucleation controlled in a manner as is described in U.S. Pat. No. 3,158,912--Schweikert. Subsequently, methods and apparatus were developed to provide such articles with an elongated grain structure as a result of directional solidification during the casting process. Such a grain structure provides improved high temperature properties predominantly in respect to thermal fatigue life over conventionally cast structures having equiaxed grain structure of the type provided through practice of the invention described by the Schweikert patent. One such method and apparatus for conducting directional solidification is described in U.S. Pat. No. 3,897,815--Smashey. Further adjustment of casting properties has been recorded in such patents as U.S. Pat. No. 3,494,709--Piearcey in connection with the generation of a single grained cast part and U.S. Pat. No. 4,033,792--Giamei et al. in connection with composite articles fabricated from a plurality of members at least some of which are of the single grained type. The disclosures of these patents are incorporated herein by reference.
Provision of a unitary cast article having a composite grained structure can provide desirable mechanical properties in different parts of the article experiencing different conditions, for example, the airfoil portion and the attachment portion of a high temperature operating turbine blade or vane.